Field of the Invention
The present invention relates to a semiconductor device and a substrate on which a semiconductor element is mounted to form the semiconductor device, particularly to a semiconductor and a substrate suitable for mounting an electrical element which is highly integrated.
In recent years, the requirements for materials for semiconductor devices have reached a new stage of strictness, as high densification and high integration of electrical elements for semiconductor elements progress. In particular, it has been required to improve the material which is formed into the substrate of the semiconductor device, for advancing durability and reliability of the semiconductor devices on which an highly integrated electrical element is mounted.
Conventionally, substrates which are made of alumina (Al.sub.2 O.sub.3) has been utilized to equip the electric elements, form electric circuits or make insulation. However, thermal conductivity of the alumina substrate is about 20 W/m.multidot.K, and it is too low to sufficiently radiate the heat generated on the electrical element toward the outside of the semiconductor device, particularly, when a highly integrated circuit such as LSIs, ICs and the like is applied to the semiconductor device.
In the above circumstances, a great deal of attention is now being paid to the realization of circuit substrates utilizing a base material formed from an aluminum nitride (AlN) sintered product with a high thermal conductivity which can cope with the densification and high integration of LSIs which has been taking place in recent years.
However, aluminum nitride exhibits strong covalent bonding and is a difficult material to sinter. Therefore, the minimum sintering temperature of 1800.degree. C. is indispensable in obtainig a finely sintered body.
In order to improve this situation, Japanese Laid-Open Patent Application No. 61-117160 discloses a method whereby low temperature sintering is provided using a raw material in which a compound of rare earth elements and an oxide of alkaline earth elements are added to an aluminum nitride powder. The sintering temperature of this method is 1,700.degree. C. or less. However, there is a tendency toward staining and the like within the sintered body. In addition, when the circuit substrate is adopted for a semiconducEor element package, there is even the problem that the sealing characteristics deteriorate in the process of glass-sealing a cap onto the base material, the aluminum nitride sintered body, of the circuit substrate.
In addition, Japanese Laid-Open Patent Application No. 82-158178 discloses the provision of high densification and high thermal conductivity in the aluminum nitride sintered body obtained by using a raw material to which at least one type of material selected from elements in group IVa, group Va, group VIa, group VIIa, and group VIII of the periodic table is added to aluminum nitride powder. However, this document discloses nothing about a circuit substrate manufactured by using the disclosed aluminum nitride sintered body or a semiconductor element packages for which the circuit substate is adopted, and the glass-sealing characteristics of this sintered body, in fact, is insufficient.
Moreover, addition of boron and an element selected from the group consisting of titanium, chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, molybdenum, cadmium, tin and tungsten is described in Japanese Laid-Open Patent Application No. 4-130064. However, the additives of this case are to improve the coloration or the transmission characteristics of the aluminum nitride, and there is no description about densification or glass-sealing characteristics of the sintered aluminum nitride.
In the above-described circumstances, even with use of a conventional aluminum nitride sintered body as described above, there is still the problem that it is rather difficult to manufacture a circuit substrate having prominent electric properties and a semiconductor package with high durability and reliability.